Motivated by economic and technical incentives, operators and crewmen of rotating machinery are awaiting with anticipation for revolutionary developments in maintenance management that would replace conventional (pre-scheduled) maintenance schemes and scheduled overhauls by a program based on established specifications. In other words, a machine element will not be taken out of service until some means of functional prediction shows that the element is defective or will be inevitably faulty within a short time and that the defect will affect the operation in total.
Conventionally, new equipment is installed, maintained, and overhauled at regularly scheduled intervals. Trade-off between the cost of unscheduled machine breakdown and the cost of unnecessary maintenance is inevitable. The ability to detect and possibly diagnose faults in rolling elements and rotating machinery has been widely employed. However, diagnostics are not enough to a priori detect symptoms of equipment failure and to avoid any subsequent damage, without prognostic capabilities. Based on information extracted from early appearing symptoms of failure, prognostics of a system can provide predictive capabilities using both trend analysis and parametric analysis.
Accordingly, there is a need for prognostic capabilities to minimize unpredicted failures, reduce maintenance costs, and increase machine availability especially in combat by the use of continuous inprocess monitoring, detecting, and indicating failure modes prior to the failures of mechanical equipment.
Based on life tests of typical mechanical equipment, a high rate of failure is observed at the start of life due to the presence of defective components. Excluding such defective lot, the failure rate maintains a constant rate for a long period of time until wear takes place and the failure rate rises at a rapid rate once a defect appears. Localized defects ultimately brings the element's life to a sudden end, since a defect starts to grow in size and induces the presence of other defects.
Accordingly, there is a need for on-line monitoring system to detect failure symptoms from inception until defect growth takes place and to identify incipient failure. This requires a combination of diagnostics and prognostics capabilities.
A warning system based on prediction of incipient failure is important since some failure modes; such as fatigue, in critical components of machinery may lead to potential faults which cannot be monitored. Also, significant increase in availability and reliability, as well as improvement in maintainability of gearboxes (and rotating machinery in general) can be achieved by developing a capability to continuously predict the remaining acceptable operating life.
Accordingly, there is a need for a system that combines diagnostic and prognostic capabilities with a warning capability that alerts the operator or pilot during flight or during equipment operation, and the inspection, testing and maintenance crew during offline maintenance to the expected acceptable and safe operation time before the next overhaul or replacement of the equipment.
In the presence of a detected defect, especially in a helicopter main gearbox, associating the size of detectable defects and the warning time (expected duration of acceptable operating time) that remains before the gearbox can no longer transmit the required operating power is necessary to assure safe and efficient performance since in the helicopter gearbox, rolling contacts play a major role in the failure. Although helicopter gearbox defect detection systems are in existence, the effectiveness of such systems is dependent upon the warning that is given prior to the complete failure of the gearbox based on predicting degradation due to causes such as wear and fatigue of typical mechanical equipment with rolling contact, such as rolling element bearing and gear of the helicopter main gearbox. The rate of growth of specific sizes and types of defects under various loading conditions relate the loading conditions to specific application and then to the expected operating life.
Accordingly, there is a need for a measure of life expectancy with appropriate confidence limits and based on the growing rate of defects to be employed in such warning system.
A variety of systems are known which employ computers and sensors in conjunction with "expert" systems, and to predict systems performance. An example of such system is U.S. Pat. No. 3,390,248 issued to Keller which relates to an impact sensing detector and provides an apparatus that monitors the operation of metal forming impact presses, wherein monitoring is done by comparing the signal amplitude against corresponding standard samples derived from normal process operation. The difference between compared samples are indications of malfunction. No stochastic time series data analysis or defect growth rates were involved and the detector is applied to equipment different from those to be monitored by the present invention.
In another example, U.S. Pat. No. 3,544,774 issued to Peklenik, relates to an apparatus for determining of a statistic characteristic magnitude in stochastic processes specially that of a fluctuating signal parameter for testing a process or condition by multiplying together signals derived from a device which calculates the standard deviation of the fluctuating parameter, a function generator which produces a constant signal which is a known function of the predetermined value, and a counter which records the number of occasions on which the parameter exceeds a predetermined value.
U.S. Pat. No. 3,697,957 relates to a self-organizing control system, the control system combines statistical decision theory, prediction theory, and rapid trial generation to determine future control actions. The system includes on-line sampling and changing of the system operation. Performance assessment units are included, and a "probability state" variable units are included for the control operation.
U.S. Pat. No. 3,962,694 issued to Calia et al. relates to a method and apparatus for monitoring of the stress level output of electrically actuated tools to ensure that the tool is operated correctly on each successive run. Thus, the apparatus prevents recycling of the tool.
U.S. Pat. No. 4,023,044 issued to Miller et al. discloses an automatic machine tool including a monitoring system and describes a system wherein the values of a test function are monitored against the value of a reference function, where the test function uses a signal produced by a transducer in response to mechanical impulses developed by operation of the machine tool.
U.S. Pat. No. 4,084,245 issued to Bunge sets forth arrangement for statistical signal analysis and uses spectral component of signals and through a comparison device, compares if the instantaneous signal values of all spectral components corresponds to the stored corresponding signal values. The function of this system is purely signal analysis using spectral components.
U.S Pat. No. 4,115,867 to Valadimirov discloses a special purpose digital computer for computing statistical characteristics of random processes utilizing stochastic data rounding units. However, this reference fails to teach the use of this circuit for detecting or indicating the condition of a systems operation.
U.S. Pat. No. 4,433,385 issued to De Gasperi relates to a method and apparatus for real time detection of false and industrial objects and describes a system for quality control using image processing methods, where the image of a sample object is taken, digitized and stored. While this reference discloses a system for the detection of faults, it does not set forth a system which uses stochastic time series signal analysis, or the detection of defect growth rate.
U.S. Pat. No. 4,472,784 issued to Blackman relates to a technique to insure independence in random sampling for data gathering purposes. However, this reference is limited to providing an improved random sampling process and does not relate to the prognosis and diagnosis of potential failures.
U.S. Pat. No. 4,587,470 relates to a multiplex control system having a backup controller. In case of an abnormal condition, control is switched from the main controller to a secondary controller. An interfacing means is provided for providing a time lag to prevent tracing of control data. The control system addresse difficulties in tracing computational data for a system including main and subsidiary controllers of different hardware structures, such as analog and digital devices in combination. Thus, this reference addresses the problem of providing two differing control structures to assure reliability.
U.S. Pat. No. 31.750 to Morrow discloses a system for monitoring malfunction of machinery by analyzing data representing machinery operation and using trend monitoring to produce an electrical signal indicative of a physical condition of the apparatus to be monitored after the occurrence of a malfunction; however, this reference is not concerned with early symptoms of degradation which may occur slowly and suddenly end in catastrophic failure, thus allowing detection of wearout at an early stage before the system shows any kind of malfunctioning. Vibration signals are used in monitoring vibration produced by machinery and are analyzed using frequency spectrum analysis after a malfunction is detected; however, spectrum analysis is not used for detection of malfunction. A succession of stored signals from any monitor which would indicate a probable malfunction of the apparatus and probable time to failure is calculated by a computer and plotted as signal amplitude versus time. Morrow further fails to disclose the use of stochastic filter elements of models.
The above-cited references fail, however, to disclose a method of monitoring wear or defect growth in rotating mechanical components using stochastic analysis which thus allows detection of degradation or incipient failure long before the component fails and which provides a warning of the remaining time to failure, as more particularly described and claimed herein.
There is, therefore, a need in the art for a reliable wearout monitor for prognostics to predict incipient failure in rotating mechanical equipment and for diagnostics of recurring symptoms as failure progresses in the equipment and for diagnostic of recurring symptoms as failure progresses in the equipment.
There is also a need in the art for a knowledge-based model of the process and components of interest, formulating an empirical time series model from measured signals, and predicting dynamic behavior of the system and its components of interest by evaluation on noise and other behavior of the signals, and using a rule-based expert system to identify the nature and location of an impending failure.